1. Field of the Invention
The present invention concerns a method for generation of magnetic resonance exposures of an examination subject, wherein a number of coils that are positioned at various locations relative to the examination subject are available in the magnetic resonance tomography apparatus that is used. Moreover, the invention concerns a corresponding control device for a magnetic resonance tomograph in order to generate magnetic resonance exposures of an examination subject according to such a method as well as a magnetic resonance tomography apparatus with such a control device.
2. Description of the Prior Art
Modern magnetic resonance (MR) systems normally operate with a number of various antennas (called coils in the following) for emission of radio-frequency pulses for nuclear magnetic resonance excitation and/or for receipt of the induced magnetic resonance signals. A magnetic resonance system normally has a larger whole-body coil that is permanently installed in the apparatus. The whole-body coil is typically cylindrical—for example having a structure known as a birdcage structure—around the patient acceptance chamber in which the patient is positioned on the patient positioning table during the measurement (data acquisition). Furthermore, one or more small local coils or, respectively, surface coils are frequently used in an MR tomography apparatus. In some examinations, a number of coil arrays each composed of one or more connected coils are often placed on and/or under the patient. These local coils serve to acquire more detailed images of body parts or organs of a patient that are located relatively close to the body surface. For this purpose, the local coils are applied directly at the location of the patient at which the region to be examined is located. Given the use of such a local coil, in many cases the magnetic resonance signals are excited using the whole-body coil (as a transmission coil) and are received with the local coil (as a reception coil).
For the generation of high-quality magnetic resonance exposures, it is without doubt important to select, from among the multiple of coils present in the apparatus, the coils that are particularly suitable for a specific measurement of a specific measurement region, i.e. for example a specific slice or a slice stack or volume within the measurement subject. This conventionally ensues manually by the input of appropriate selection commands at a control terminal of the tomography apparatus. The operator makes his or her selection depending on whether the appertaining coil is located in a suitable position relative to the region to be acquired in the subsequent measurement and exhibits a matching exposure region, i.e. whether the region of interest can actually be measured with this coil.
For coils with a fixed position with regard to the patient positioning table, this position is in part explicitly set at the manufacturer's facility. This position is then in principle known to the magnetic resonance system, i.e. in the control device of the magnetic resonance tomography apparatus, even though this coil normally can be shifted within a small range. As an alternative, the position can be explicitly measured before the magnetic resonance measurement. A method for determination of a coil position is, for example, specified in German OS 102 07 736. A magnetic field gradient is thereby applied in the appertaining spatial direction in which the position of the coil should be determined. A radio-frequency signal is then emitted with the whole-body coil or with a local coil, and a reception signal profile is measured in a direction along the magnetic field gradient by means of the appertaining local coil whose position is to be established. Finally, the position of the local coil is determined in the appertaining spatial direction with the aid of this signal profile. In addition to being received by the local coil to be localized, the reception signals are received by an antenna with a homogeneous sensitivity over the entire measurement space, for example by the whole-body coil. The intensity values received by the local coil are normalized with the intensity values received by the antenna with homogenous sensitivity. A function that is approximated to the shape of the sensitivity profile of the local coil is adapted to the spatial curve of these normalized intensity values. The maximum point of this function is then adopted as the position of the local coil.
Generally, only the position in the z-direction (i.e. in the longitudinal direction of the patient positioning table) is measured (detected). As before, the coordinates perpendicular to this are unknown and, with some manufacturers, are approximated at the factory to an average expected value. An exposure region can likewise be specified at the factory, but this is merely a estimated exposure region to be expected on average. In particular, it is not considered whether this region is also actually filled by a load in a measurement or whether, due to the load, the exposure region has a very different shape than the specified (for example rectangular) shape.
The correct selection of the coils thus requires a significant degree of knowledge and experience on the part of the operator, particularly since the available information specified at the factory about positions and exposure regions of the coils is often not sufficiently precise in practice and does not take into account the real factors for the actual measurement. Insofar as the optimal coil or coil combination is not selected for a subsequent measurement, the quality of the subsequent exposures is also unavoidably worsened. This can possibly lead to scans having to be repeated, which extends the total exposure time. This not only reduces the efficiency of the magnetic resonance tomography apparatus and of the operating personnel, but also most notably leads to a higher exposure of the patient.